In doppler radar systems, it often is necessary to obtain information which indicates the presence or absence of a moving target--often mixed with clutter--in each doppler frequency resolution cell, that is, for each narrow band of doppler frequencies.
Each moving target in a dopler radar system is represented by a single side band doppler signal corresponding to its radial speed. An upper sideband would be obtained for an approaching target and a lower sideband for a receding target. Doppler signal returns owing to scatterers, for example, wind-driven foliage in a forest or jungle would have an oscillatory motion, and the doppler frequency returns, if integrated over a sufficient time interval, would have substantially equal energy in both sidebands.
In order to cancel the energy in both sidebands to reduce the effect of returns from such scatterers, a system was developed by Otto Rittenbach and described in U.S. Pat. No. 3,422,430, issued Jan. 14, 1969. Such a system, while useful for doppler returns over a broad band of frequencies, does not lend itself well to applications requiring high doppler resolution for low doppler frequencies, unless one were to incorporate narrow band low frequency filters in the system of U.S. Pat. No. 3,422,430. However, it is difficult to construct such narrow band low pass filters.